1. Field of the Invention
The present invention relates to a golf club head which is excellent in directionality of a hit ball.
2. Description of the Related Art
In accordance with an increase of a head volume in recent years, a moment of inertia around a center line of a shaft axis is increased in a golf club. The moment of inertia around the center line of the shaft axis has a close relation to a return of the head during a swing. For example, Japanese Unexamined Patent Publication No. 2001-299968 teaches that when the moment of inertia around the center line of the shaft axis becomes great, it becomes easy to hit the ball in a state where a face is open at a time of swinging. As a result, a miss shot called a “slice” tends to occur.
Further, the large size of the head volume has an inclination to make a depth of center of gravity of the head large. In Japanese Unexamined Patent Publication No. 11-33145 (1999), there is suggested that the larger the depth of center of gravity of the head is, the better the directionality of the hit ball is.
Further, the large-sized head has an inclination to make a moment of inertia around a vertical axis passing through the center of gravity of the head, in addition to the moment of inertia around the center line of the shaft axis. In the swing process, if the hitting face of the head can be positioned in an address state, such a head can hit the ball with a small displacement with respect to the intended direction even when the head hits the ball by a portion of the hitting face close to a toe or a heel. Accordingly, conventionally, it is sufficient that the large-sized head has a magnitude of the depth of center of gravity which is consequently defined on the basis of the head shape, so that there is no idea for further improving the depth of center of gravity.
The present inventors have measured the depth of center of gravity of the various heads and the moment of inertia around the center line of the shaft axis. The result of measurement is shown in FIG. 5. In FIG. 5, the conventional head is plotted by black dots. It is known from this graph that the head having the great moment M of inertia around the center line of the shaft axis has a great depth L of center of gravity.
However, taking a view of swing of golfers, most of the golfers belong to the following two types:
(a) a type that a head speed is high in a swing initial stage and is low in a swing later stage; and
(b) a type that a head speed is increased from a swing initial state to a swing later stage.
Further, taking a view of a certain golfer, there is a case where the type (a) and the type (b) alternately appear per swing. In particular, since a beginner golfer has no stable head speed, the beginner golfer develops a strong tendency to the above. As a result, the beginner golfer has an inclination that the direction of the hit ball is not stable. Further, the greater the moment of inertia around the center line of the shaft axis in the head is, the more the above inclination is occurred.